TOPIC 9. STRUCTURE DETERMINATION (chapter 9 and part of chapter 2)

OBJECTIVES 1. Use combustion analysis to determine empirical formula. 2. Determine molecular weight (and molecular formula) from mass spectrometry. 2. Calculate number of rings and double bonds from molecular formula. 3. Determine functional groups present from infrared spectroscopy. 4. Use 1H and 13C NMR spectroscopy to identify other structural features. 5. Combine conclusions from individual techniques to determine the structure of organic compounds. Problems in this section will be restricted to the following classes of aliphatic and aromatic compounds: Alkanes, alkenes, alkynes, alkyl halides, alcohols, ethers, aldehydes, ketones, carboxylic acids, esters, acyl chlorides, anhydrides, amides, amines, nitriles

COMBUSTION ANALYSIS CxHyOz + O2 xCO2 + (y/2)H2O Measure mass of CO2 and H2O formed from a known mass of compound; data cited as mass % of each element present. e.g., C H O Mass% 55.7 7.0 37.3 Mole% = Ratio Be careful when rounding. Do not round by more than 0.05 to 0.1

EMPIRICAL AND MOLECULAR FORMULAS; DETERMINATION OF “SODAR” Molecular formula is an integral number of times the empirical formula. C2H3O Sum of double bonds or rings (“SODAR”) For C,H,O: (2#C + 2 – #H) / 2 For C,H,O,N,Hal: (2#C + 2– #H – #Hal + #N) / 2 Each Hal replaces a H Each N adds an extra H S,O: no effect on calculation If SODAR calculated from empirical formula is not a positive integral (or 0), this cannot be the molecular formula.

Problem: Determine the value of SODAR for the following molecular formulas. CH3Br CH5N C3H8N C6H6

MASS SPECTROMETRY Theory and Experiment M + e– (70 eV) M+ + 2e– M+ lower mass ions

Data Available e.g., mass spectrum of ethane, CH3CH3 abundance Ions also fragment, which can give further clues about the structure (beyond scope of this course). Knowledge of molecular weight (from mass spec) and empirical formula (from combustion analysis) allows determination of molecular formula. abundance 0 10 20 30 m/e

SPECTROSCOPY: THE INTERACTION OF LIGHT AND MATTER

INFRARED SPECTROSCOPY See Chapter 2; Table 2.7 COOH 3100 sp2C–H 1600 C=C 3500, O-H 1000-1200 C–O 2900 sp3C–H 1700 C=O 4000 600 Wavenumber / cm-1

Problem: Determine the structure of the compound with the following empirical formula and IR spectrum. Empirical formula: C4H8O IR; peak at 1720 cm-1; no broad peak at 3300, no strong peaks at 1000 to 1200 cm-1, no strong peaks at 1600-1650 cm-1